1. Field of the Invention
The present invention relates to fuses of a small physical size, and more particularly, this invention relates to miniature plug-in fuses, such as those utilized in automobiles, having conductors and a fuse link stamped from the same strip of fusible metal.
1. Description of the Prior Art
To prevent undesired heating and potentially damaging and dangerous overload conditions, a fuse or fuse link is inserted in series with a conductor so that it will melt at a desired current level. Such a fuse link is usually made of a metal or alloy (fusible metal) which has a significantly lower melting point than the copper conductors.
In general, a fuse should be of the smallest physical size permitting required levels of current flow and yet assuring interruption of the flow of electrical current when desired. Various types of fuses, including some for interrupting relatively low levels of current in relatively low voltage applications, have been used. The most prevalent type of fuses in the latter category are those having a sealed glass cylindrical envelope surrounding a fusible link which is electrically connected to the terminal connectors capping each end. This type of fuse is in common use in automobiles. Such a fuse suffers from several disadvantages. It is comparatively bulky and fragile; it is difficult to handle; and it has its terminals exposed so that undesired contact is possible.
Less bulky fuses have been developed, but in the process of doing so, additional drawbacks were introduced. Some of these fuses had links exposed where they could be contaminated. Also, a user trying to replace a blown fuse was subject to risk of electrical shock or injury from contact with the hot melted fusible metal. Other types enclosed the fuse link and provided a handle, but were difficult to extract because of their compactness. In addition, the fuse link could not be visually examined while the fuse was in a terminal block (and in some cases could not be visually inspected at all). Further, these fuses were not economical. Many of the prior art types of fuses involved soldering the fusible link to the terminal connections, which were often of a dissimilar metal. In such fuses, corrosion or deficient soldering techniques frequently results in hot spots at the solder connection, causing an undesirable alteration of the current response or "blowing" characteristics of the fuse. Since fuses are selected by a designer on the basis of the fuse "blowing" characteristic (i.e., the time required to interrupt current flow at given voltage and current levels), any variation in the fuse blowing characteristic is particularly undesirable.
Another attempt to eliminate or diminish many of the drawbacks and difficulties of prior art devices involves constructing the conductive elements entirely of fusible metal. The terminal connections are in a coplanar configuration and the conductive elements are partially enclosed in a transparent plastic housing open at the end from which the terminals (or plug-in members) extend. One type of such a fuse is made by Littelfuse, Inc., and disclosed in a number of patents (e.g., U.S. Pat. No. 3,909,767--Williamson et al.; U.S. Pat. No. 3,962,782--Williamson et al.; U.S. Pat. No. 4,023,264--Schmidt, Jr. et al.; U.S. Pat. No. 4,023,265--Aryamane; and U.S. Pat. No. 4,040,175--Williamson et al.). While such fuses are useful, there are areas in which improvement is desirable. A number of these stem from the open housing at the terminal end. The fusible link is not wholly enclosed and thus can easily become contaminated, such as by becoming coated with foreign matter. Any coating of the fuse link alters the heat transfer characteristics, and may alter the conductive characteristics, of the fuse link. Such an alteration will necessarily alter the blowing characteristic of the fuse and result in melting of the fuse link at undesired current levels, either too high or too low. Further, the open housing design is unable to produce any substantial pressurization during blowing of the fuse link, which would enhance arc snuffing; nor will the housing retain the melted fusible link metal within the fuse. Further, the open housing does not provide as much support for the protruding plug-in members or terminal portions as a closed structure, so that misalignment is more likely.
Another problem relates to the fact that the insulating housing of the fuse may be strengthened by a protruding shoulder surrounding its outer closed end. In some types of fuse assemblies, this shoulder is essentially at right angles to the sides of the housing, and when the fuse is inserted in a terminal block, conductors in a harness may be introduced between the shoulder at the end of the fuse and the terminal block. When this occurs, the fuse can be inadvertently dislodged from the block by manipulation of the harness while wiring.
The method of manufacturing a miniature fuse with an open plastic housing and conductors and fuse link progressively formed from a continuous strip of fusible metal, the conductors being surrounded by the housing and secured to the housing by causing the plastic to flow through apertures in the fuse metal, is relatively economical. However, the manner of affixing the housing to the fuse metal has some drawbacks in that the necessity of heating the plastic to bond or to flow, or both, limits the rate at which fuses may be produced to considerably less than the rate that is achievable with metal working operations. To achieve rates comparable to metal working operations, it is necessary to have a multi-stage operation, adding complexity to the method, or multiple process lines. A related, more serious, drawback of this method of affixing the housing to the fusible metal is that the state of technology is such that, compared to metal forming, the reliability of the fastening is limited and separation of parts may be encountered by the user.